This invention relates to circuit breaker systems and, more particularly, to an arc-fault detecting circuit-breaker system responsive to the occurrence of arcing in a protected electrical circuit.
Aircraft electrical systems are normally protected from high current and electrical shorts by circuit breaker devices. These circuit breakers are designed to remove power from a protected electrical circuit element if an electrical current above a preset value is passed through the device. These high currents may occur for a number of reasons, such as a failure occurring in a piece of electrical equipment or damage occurring in a section of wiring insulation allowing the conductor to come into electrical contact with the structure of the aircraft, which is normally at ground potential.
Due to the nature of the circuit breaker mechanism, the tripping of the circuit breaker is not always xe2x80x9cinstantaneousxe2x80x9d. Some types of circuit breakers are allowed to continue supplying current for up to 40 seconds at twice their specified trip current. An overload of five times the rated current is allowed to flow for up to three seconds before a trip must occur. These trip delays are allowed because these devices rely on the overcurrent to heat up a bimetallic strip that functions as the detection element within the circuit breaker.
Circuit breakers conforming to these requirements have been used in aircraft for many years. Under normal operating conditions and under normal fault conditions, they operate satisfactorily. However, there are some fault conditions where the tripping delay greatly affects the ability of the circuit breaker to protect life and property. For example, certain types of wiring failures allow for a fault to ground which is not a xe2x80x9cdead shortxe2x80x9d, meaning a direct, virtually zero-resistance electrical connection to ground. Certain types of wiring insulations arc track when electrically faulted, which locally turns the material from an insulator to a conductor. An arc-tracked wire can be shorted to ground through a resistance which serves as a current limiter, which in turn allows the current to flow through the wire to the fault for some time until the circuit breaker is tripped. Until that occurs, the high current flow can damage and arc track other wires, adding their electrical supplies into the fault. This fault may initiate a cascading chain reaction which quickly compromises the safety of the aircraft.
These types of events occur sufficiently often in aircraft wiring systems that there is a need for a device which can detect arcing faults and remove electrical power from the protected electrical circuit element more quickly than can a standard circuit breaker. In addition, such a device must meet other requirements, such as space limitations, low cost, and xe2x80x9cinvisibilityxe2x80x9d to normal operating conditions of aircraft electrical systems. The present invention fulfills this need, and further provides related advantages.
The present invention provides an arc-fault detecting circuit-breaker system and a method for its use. The arc-fault detecting circuit-breaker system responds both to excessive currents, as does a conventional circuit breaker, and to arc faults (sometimes termed xe2x80x9csparksxe2x80x9d). These overcurrent and arc-fault-responsive functions are combined into a single arc-fault detecting circuit-breaker element that allows normal functioning of a protected electrical circuit element under ordinary operating circumstances, but responds instantaneously when a fault occurs. It may be used in circumstances where conventional circuit breakers are now used, but adds the additional capability of arc-fault detection. The arc-fault detecting circuit-breaker system is highly reliable, but fails to a safe state if failure should occur. It is light in weight and small in volume, may be packaged in a manner similar to that of conventional circuit breakers, and is relatively inexpensive. It does not affect the normal operations of the protected electrical circuit element.
In accordance with the invention, an arc-fault detecting circuit-breaker system comprises a normally closed line circuit breaker in series with a protected electrical circuit element (including an electrical load and the wiring line leading to it) whose current flow is to be interrupted upon the occurrence of an overcurrent condition or an arc fault. It further includes a detector of the rate of change with time of the current flow in the protected electrical circuit element, and a circuit-breaker activating (tripping) element operable responsive to the detector. The circuit-breaker activating (tripping) element opens the circuit breaker in the event that the detector detects a rate of change of current with time in the protected electrical circuit element in excess of a permitted maximum rate-of-change value.
The line circuit breaker is preferably a resettable circuit breaker, but it may be a one-shot fuse circuit breaker.
The circuit-breaker activating (tripping) element preferably includes a silicon-controlled rectifier (SCR) and, optionally, a resistor in the SCR gate circuit. Arc-fault detecting circuit breaker systems used in direct current (DC) applications may be made with a single SCR. Arc-fault detecting circuit breaker systems for alternating current (AC) applications use two SCRs to operate with the reversing voltage potential. An arc-fault detecting circuit breaker system built for AC applications will operate properly in DC applications as well.
For the DC application, the SCR is connected from the line to ground. For the AC application, the second SCR is connected in opposite polarity from the line to ground. Whether in AC or DC operation and whether the circuits have return wires or are locally grounded, the SCRs carry current from the line to ground (or vice versa) when activated. This type of connection, termed a xe2x80x9ccrowbarxe2x80x9d circuit, causes a large current to flow through the normal current-limiting line circuit breaker. The line circuit breaker activates or xe2x80x9ctripsxe2x80x9d, halting the flow of current through the line circuit breaker and therefore also to the load. In addition, the SCR(s), by providing a low-resistance path to ground compared to the line or line fault, virtually instantaneously stops the flow of current to the fault which initiated the event, limiting further damage.
The arc-fault detector triggers the operation of the SCR(s). The detector may have various forms. It may be either magnetically saturable or non-saturable in design and operation.
In one form, the detector comprises a conductive shield (such as a braided copper wire) which surrounds the electrical line that forms part of the protected electrical circuit. This structure may in turn be contained within a magnetically permeable tube that can be saturable. This construction has a natural built-in air gap for handling direct currents and provides a low inductance for reduced sensitivity to normal circuit variations that might induce false triggers. Low inductance and sensitivity make this form of the detector particularly useful for high DC current circuits.
A second type of small, magnetically efficient detector may be constructed using ferrite pot-cores of nickel-based magnetic materials which saturate readily. This small size makes it easy to incorporate the detector into a circuit breaker package. Continuous currents of 20 to 30 amperes may be handled with a small resistive loss.
The primary inductance of the detector is small. Only abnormally large low-frequency transients or optimally faster, low-current arcing events will trigger the SCRs. These low-value arcing currents occur as an addition to normal primary current and force the detector""s inductor core toward saturation. A saturable reactor produces voltage pulses (LdI/dt) of opposite polarity as its locus of operation enters and leaves the region of core saturation. A large fault current holds the core in saturation for a large fraction of a fault-event time. This behavior implies a short duty cycle, with increasing inductance, as the fault current decreases. There is therefore a large back emf (trigger pulse). There is similarly a short duty cycle going into saturation. This can have the effect of charging the SCR gate capacitance sufficiently to keep the device in a conduction mode when its anode voltage is cyclically removed. The SCR devices chosen for this application are preferably constructed with internal shorts such that the anode and gate voltages during conduction, with respect to the cathode, are almost equal.
All nominally identical semiconductor devices have a variation or spread of operating parameters. The circuit-breaker activating elements may be made to function more identically in respect to external circuitry if appropriate series current feedback is used. In this application, an impedance (series resistor) may be inserted into the gate drive line. The series resistor has a value of several times the effective gate dynamic resistance at the trigger point. The price of this feedback is a larger applied drive voltage from the detector. Larger values of series resistor lower the sensitivity of the circuit. If the sensitivity is too high, then triggering may occur with a normal power-up or breaker re-set.
The present arc-fault detecting circuit-breaker system is operable to detect both conventional overcurrent conditions and also arcing conditions, and to interrupt the circuit of the protected electrical circuit element upon the occurrence of either type of condition. In the case of an arcing condition, the system interrupts the circuit much more quickly than would a conventional circuit breaker. The sensitivity of the detector is set such that normally expected electrical transients, such as startup voltage spikes and motor startup loads, do not trigger the SCR(s). The device will not misinterpret these conditions as arcing events, and trip the breaker and remove power from some system inadvertently. The latter is an important consideration for aircraft operations.
Surge currents and power supply switch-on transients may cause substantial transients in a circuit yet still be normal in such applications, see MIL-STD-704. These currents may have large peak values but are generally bounded by an upper frequency limit close to 1000 Hz (Hertz). The arc-fault detecting circuit breaker of the invention desirably operates with very small values of inductance, typically less than 100 microHenrys, so that its responsiveness is greatly diminished at frequencies on the order of 1000 Hz and below. The present approach has a normal circuit breaker element in series in the protected circuit, which is designed to accept and not respond to such transients. Thus, the present invention does not trigger when exposed to normal surges and transients in current.
The arc-fault detecting circuit-breaker system is operable to detect current changes that are smaller than the normal operating current, such as a beginning arc at the end of a long wiring harness. This capability has great significance for 400 Hz operation, such as found in many aircraft electrical systems, where wiring impedance due to the length of the wiring harness may naturally limit fault current flow even though an arcing event (or even a fire) is in progress.
In its preferred embodiments, the arc-fault detecting circuit-breaker system is light in weight and relatively inexpensive, so that it may be used to protect many circuits in aircraft flight applications where weight is an important consideration. It is also small in size, so that it may be used in new circuits, or retrofitted into existing circuits, without exceeding space constraints. The small size also permits it to be packaged into about the same package size as conventional line circuit breakers. The arc-fault circuit breaker system of the invention may be configured as an add-on supplement to an existing circuit breaker if replacement of the circuit breaker is not possible. The detector or even all of the device other than the circuit breaker itself may be remote from the circuit breaker, such as at the end of a long wiring harness.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.